1. Field of the Invention
The present invention relates to a solid state image pickup apparatus and, more particularly, to a system for reading optical information accumulated in photoelectric converting elements.
2. Related Background Art
FIG. 1 is a schematic circuit diagram of a conventional solid state image pickup apparatus.
In the diagram, electrodes 100a of photoelectric converting elements 100 arranged like a matrix are commonly connected to a horizontal signal line 101 for every row. Each horizontal signal line 101 is connected to an output terminal of a vertical scanning circuit 102. Electrodes 100b of the photoelectric converting elements 100 are commonly connected to a vertical signal line 103 for every column. Each vertical signal line 103 is commonly connected to a read signal line 105 through a horizontal scanning transistor 104 having an MOS structure. A gate electrode of each scanning transistor 104 is connected to each output terminal of a horizontal scanning circuit 106. The scanning transistors 104 are sequentially turned on when a horizontal scanning pulse from each output terminal is shifted. The optical information accumulated in the photoelectric converting elements connected to a certain horizontal signal line are serially read out to the read signal line 105 by the horizontal scan. The readout optical information are amplified by an amplifier 107 and output to the outside.
A MOS transistor 108 connected to the read 105 is used to clear the signal line capacity of the read signal line 105.
In such a conventional technique, there is a fear that the level of output signal may decrease due to the capacity of the read signal line. In particular, when the number of horizontal scanning transistors increases and they are arranged at a high density, the capacity of the read signal line increases and the signal level remarkably decreases.
Therefore, there is considered a method whereby the decrease in output level is avoided by providing a buffer circuit. However, in this case, it is necessary to alternately read out the signal and noises in order to correct the noises which are generated from the buffer circuit. This causes a frequency of the scanning pulses to become high. In addition, there is also considered a case where the frequency of scanning pulses becomes high due to the realization of high resolution.
There are problems such that when the frequency of scanning pulses becomes high, a limitation on design becomes severe and the circuit arrangement also becomes complicated.
Therefore, there has also been proposed a system (JP-A-61-154366; Japanese Laid-Open Patent Gazette No. 61-154366) in which the read signal lines are divided and the vertical signal lines of the odd-number columns and the vertical signal lines of the even-number columns are connected to the individual divided read signal lines. With this construction, although the foregoing problems can be solved, the read signal lines are unconditionally determined for the photoelectric converting elements of the odd-number columns and for the photoelectric converting elements of the even-number columns, so that there is a fear that the application range is narrowed in the application field. For example, in the case of using the apparatus as an image pickup device is a black and white camera, it is advantageous to read out the signals of the same horizontal signal line from the same read signal line in terms of the characteristics and the installing efficiency. On the other hand, even in the case of using the apparatus as an image pickup device in a color camera, although there is a situation such that it is necessary to change the read signal lines in dependence on an arrangement of color separation filters, such a change cannot be performed in the foregoing conventional technique.
There has also been proposed a system (U.S. Pat. No. 4,658,287) in which MOS transistors are arranged at both ends of the same vertical signal line and connected to the read signal line. However, according to this invention, in order to enable one image pickup device to be used for either a black and white (B/W) camera or a color camera, an output signal line for the B/W camera is merely provided on one side of the vertical signal line and an output signal line for the color camera is merely provided on the other side.